1. Field of the Invention
The present invention relates to a color cathode-ray tube with a shadow mask.
2. Description of the Related Art
In a color cathode-ray tube, electron beams emitted from an electron gun pass through apertures formed in a shadow mask, and then strike a phosphor screen, thus causing phosphors to emit light.
As shown in FIG. 9, a shadow mask 95 is welded to a mask frame 96 in such a manner that tension is applied in a direction indicated by arrows 9 (a vertical direction, i.e., a Y-axis direction). The shadow mask 95 is provided with a large number of apertures 90, through which electron beams pass and reach a phosphor screen.
In such a tension-type shadow mask 95, the apertures 90 formed in the shadow mask 95 are shaped and arranged as follows. In general, a large number of substantially equi-shaped slot apertures 90 are aligned in such a manner that their longitudinal directions correspond to the Y-axis direction as shown in FIG. 10. Moreover, it is suggested that, for preventing breaking of the shadow mask and improving brightness, two kinds of apertures 90a and 90b having different lengths in the Y-axis direction are aligned in combination as shown in FIG. 11 (e.g., see JP63(1988)-43241A).
Since the electron beams are blocked by bridges 91 partitioning off two apertures 90 (or 90a, 90b) adjacent in the Y-axis direction in FIGS. 10 and 11, shadows are formed on the phosphor screen due to the bridges 91. Phosphors do not emit light in the shadow portions, so that the shadow portions become non-light-emitting regions. The presence of such non-light-emitting regions decreases the brightness of a displayed image.
Thus, in order to improve the brightness of a displayed image, it is preferable that the pitch in the Y-axis direction of the bridges 91 is increased to decrease the number thereof.
When the pitch in the Y-axis direction of the bridges 91 is increased, the spacing in the Y-axis direction of the shadows of the bridges 91 also is extended. Therefore, the shadows of the bridges are likely to be recognized, and the shadows dotted over the entire screen look continuous in the horizontal direction, and are recognized as black horizontal streaks.
On the other hand, when electron beams illuminate phosphor stripes, about 20% of the electron beams that have struck a shadow mask pass through the apertures of the shadow mask, so that the remaining 80% of the electron beams heat the shadow mask to expand it thermally. In the tension-type shadow mask 95 shown in FIG. 9, the tension applied in the Y-axis direction can absorb the thermal expansion in the Y-axis direction. However, the thermal expansion in the X-axis direction is transmitted via the bridges 91, so that the positions of the apertures 90 of the shadow mask are displaced in the X-axis direction, whereby so-called doming occurs. Due to this doming, the electron beams that pass through the apertures 90 cannot illuminate desired phosphor stripes on the phosphor screen, and so-called color displacement occurs.
In order to solve the above-mentioned problem, the transmission of thermal expansion in the X-axis direction only needs to be reduced, and it is considered that such reduction can be realized by reducing the number of the bridges 91 contributing to the transmission of thermal expansion. However, the reduction in number of the bridges 91 means the increase in pitch in the Y-axis direction of the bridges 91, which causes the above-mentioned horizontal streak pattern to be generated.
Thus, in a conventional shadow mask, it has been difficult to achieve both the reduction in a horizontal streak pattern and the prevention of color displacement due to doming.